Smoke detectors of this kind are also known as fire alarms. They typically comprise a detector housing with at least one smoke-inlet aperture and a detector unit accommodated in the detector housing for smoke detection. The detector unit preferably comprises an optical measuring chamber that is shielded against ambient light but permeable to smoke to be detected. The latter also usually comprises a plurality of louvers to shield ambient light and is therefore also called a labyrinth.
An electronic control unit is connected to the light-emitting diode and the photosensor as part of the smoke detector. The control unit is configured to output a warning and/or an alarm if a respective minimum concentration value of smoke can be detected.
Also known from the prior art are smoke detectors that use two differently colored light-emitting diodes and a photosensor in one or two scattered-light arrangements. It is known to use a red-luminous LED or an infrared LED to emit red or infrared light and to use a blue- or violet-luminous LEDto emit blue or violet light. By means of a suitable evaluation of the respective colored scattered light received from the photosensor, such as, for example, ratio calculation, it is then possible to perform an evaluation with respect to the particle size of the smoke particles detected. A suitable assessment of the particle sizes determined enables, for example, a differentiation to be made between smoke, dust and steam. This enables the output of a possible false alarm to be prevented.
Known smoke detectors are typically configured for operation in a line of alarms with a plurality of further smoke detectors connectors thereto or for battery-supported stand-alone operation. This means in both cases that only a very low average electrical power of less than 10 mW is available. The control of the light-emitting diodes for the emission of the respective light is therefore typically pulsed. Similarly, the entire “electronics” are designed for the lowest possible power consumption.
Also known from the prior art is the use of two monochrome light-emitting diodes arranged side-by-side the emitted light from which is converged on a common optical axis, for example via a Y-shaped optical collecting element by or by means of two converged light guides.
The use of one single light-emitting diode with two LED chips arranged side-by-side with different colors is known from the applicant's still unpublished European patent application 14155048.3. The first chip preferably emits red or infrared light. The second chip preferably emits blue or ultraviolet light. With this arrangement, a chip axis extending through the two LED chips is parallel to the plane defined by the optical axis of the photosensor and that of the light-emitting diode. The side-by-side arrangement of the two LED chips on a common carrier also results in two optical axes differing from one another by about 15° to 25°. Hence, the scattered-light arrangement therein has two different scattered-light angles.